The ST5 proteins are novel components of growth regulatory signal transduction pathways that participate in the regulation of cytoskeletal alterations associated with the transformed phenotype. Three proteins, p126, p82, and p70, derive from a single gene on human chromosome 11p15.3. Alternative splicing distinguishes the two larger proteins; p82 lacks a segment of 420 amino acids present only in p126. This 420 amino acid segment interacts with the c-abl and MEK2 proteins. The third ST5 gene product, p70, arises from a transcript initiated at a unique promoter located within an intron of the ST5 gene, and consists of the C-terminal 609 amino acids shared in common by the three proteins. The C-terminal segment shared by the three proteins binds to the transferring receptor both in vitro and in vivo. The expression of p70 shows a striking pattern of differential regulation. In a variety of cell lines examined, p70 expression correlates with the untransformed or non-tumorigenic phenotype and corresponding characteristic cytoskeletal morphology. Expression of p70 in certain transformed 3T3 cell lines suffices to revert the transformed morphology of these cells. The major ST5 gene product, p126, functions as an activator of MAP kinase in response to growth factor stimulation. Co-expression of p70 inhibits this activity. The long term objective of the proposed research is to define the roles of the ST5 proteins in signaling and tumor suppression. The first specific aim involves systematic mutagenesis of the ST5 cDNAs to define regions of the protein required for each known activity. Mutants will be tested for their ability to activate MAP kinase, to induce cytoskeletal alterations, and to bind associated proteins. In Aim 2, experiments are proposed to determine the signaling specificity of the ST5 proteins. The ability of p126 to stimulate, or p70 to inhibit, signaling through different pathways will be assessed. In Aim 3, associated proteins will be characterized for their effects on signaling through the ST5 proteins. Aim 4 addresses the mechanism of p70 mediated inhibition of signaling through p126. Two models for p70 function are proposed and strategies to test these model are described. In Aim 5, the possibility that ST5 proteins are phosphorylated in vivo will be examined. If phosphorylation is detected, the effects of growth factor stimulation and cell cycle progression on the phosphoryltion state will be assessed.